1. Field of the Invention
The present invention relates to optical fibers used in optical communication systems, and relates in particular to an optical fiber suitable for use in high-speed and large capacity communication systems based on the wavelength division multiplexing (hereinafter abbreviated to “WDM”) method.
2. Description of the Related Art
Capacity of optical transmission system has been increasing significantly using the WDM method. In the WDM method, reduction of non-linear effects and control of chromatic dispersion are required in transmission optical fibers.
In general, the non-linear effects in an optical fiber is represented by n2/Aeff, where n2 is a non-linear refractive index of the optical fiber and Aeff is an effective core area of the optical fiber. Therefore, the non-linear effects is inversely proportional to Aeff. Accordingly, various optical fibers are developed such as optical fibers having enlarged effective core area Aeff, optical fibers having reduced dispersion slope, and optical fibers compensating dispersion slopes.
In order to increase the transmission capacity based on the WDM method, two methods are mainly used. The first method is a method of increasing the number of waves for multiplexing, and the second method is a method of improving the transmission speed.
As the method of increasing the number of waves for multiplexing, there is a trend of broadening the wavelength band for transmission. A 1550 nm-band is mainly used as a wavelength band for the WDM method. In the 1550 nm-band, a band known as the C-band (conventional band, 1530-1565 nm) has been widely used, but in recent years, there has been a trend of the use of the L-band (long wavelength band, 1565-1625 nm) and S-band (short wavelength band, 1460-1530 nm) for communication.
Therefore, various optical fibers are proposed such as optical fibers for use in C-band and L-band, and optical fibers having larger chromatic dispersion for use in S-, C- and L-bands.
However, refractive index profile in all the conventional optical fibers are designed to achieve desired optical characteristics in the vicinity of a certain chromatic dispersion. FIG. 14 is a graph showing chromatic dispersion characteristics of typical optical fibers for the WDM. Optical fibers A and B having individual chromatic dispersion characteristics are realized by using individual refractive index profile to the optical fibers.
A method for manufacturing dispersion shifted optical fibers in order to suppress the non-linear effects in low level is disclosed in a Japanese Unexamined Patent Application, First Publication No. Hei 8-220362. The invention disclosed in the Japanese Unexamined Patent Application, First Publication No. Hei 8-220362 discloses a method for designing an optical fiber having an enlarged mode field diameter (hereinafter abbreviated to “MFD”) in the 1.55 μm-band. As shown in a reference document (The Institute of Electronics, Information and Communication Engineers, IEICE Technical Report, Optical Communication Systems, OCS-94-74, Nov. 18, 1994) relating to the above patent application, in the design region of MFD enlarged optical fibers shown in the application, the chromatic dispersion and MFD vary monotonically as the core radius changes.
As a result, if the core radius is adjusted using an optical fiber core rod having a refractive index profile designed according to the method of the above patent application so as to have a chromatic dispersion different from a predetermined chromatic dispersion, it is unavoidable to occur a large change in the MFD.
When an optical fiber prepared according to the above method causes the decrease of the MFD of the optical fiber, it is not only disadvantageous in view of splice loss, but also, when the optical power density transmitted into the optical fiber is high, as in optical amplifiers, problems are occurred such as increase of non-linear effects and degradation of the transmission characteristics.
On the other hand, it is also required to reduce the dispersion slope in optical fibers for the WDM. Reducing the dispersion slope enables the small chromatic dispersion variation over a broad wavelength bandwidth. This is extremely important in high-speed transmission systems, in which the chromatic dispersion severely limits the transmission distance. In conventional optical fibers, chromatic dispersion slope typically has higher than 0.05 ps/nm2/km, but in high-speed transmission systems, it is required to provide optical fibers having a dispersion slope of 0.05 ps/nm2/km or less.